High frequency heating apparatus



G. C. LIMA Jan. 30, 1968 HIGH FREQUENCY HEATING APPARATUS 2 Sheets-Sheet 1 Filed Dec. 11, 1964 INVENTOR. GREGORY C. LIMA AGENT G. c. LlMA HIGH FREQUENCY HEA' IING APPARATUS Filed Dec. 11, 1964 T0 MAGNETRON i OUTPUT TO MAGNETRON OUTPUT 2 Sheets-Sheet 2 TO MAGNETRON E OUTPUT TO MAGNET OUTPUT INVENTOR. GREGORY C. LIMA United States Patent 3,366,769 HIGH FREQUENCY HEATING APPARATUS Gregory C. Lima, Brentwood, N.Y., assignor to North American Philips Company, llnc., New York, N.Y., a corporation of Delaware Filed Dec. 11, 1964, Ser. No. 417,588 21 Claims. (Cl. 219-1055) This invention relates to high frequency heating apparatus, and more particularly to a high frequency oven having improved means for coupling the radio frequency energy into the object to be heated.

In the microwave cooking art, the problem arises of cooking foodstuffs having a relatively large total area or volume. In order to accomplish the desired heating result, it is necessary to energize large regions of space with microwave energy, and for proper cooking the energy distribution should be substantially uniform throughout the space. When utilizing a microwave oven of the resonant cavity type, the microwave energy that penetrates the food is the result of direct radiation incident on the surface of the food and indirect energy refiected from the walls of the oven. The microwave energy supplied to the oven cavity sets up a particular energy distribution pattern throughout the cavity which is determined by the frequency of the microwave energy source and the dimensions of the cavity. This energy pattern or mode of operation is represented by a standing-wave pattern characterized by a plurality of nodes and antinodes. The resultant energy distribution exhibits corresponding maxima and minima and results in a non-uniform field within the cavity whereby uneven heating of the foodstuffs will occur.

Various solutions have been suggested to minimize or eliminate the undesirable effects that such uneven field distributions may have on the heating of the material to be cooked. It has been proposed to excite the oven in a large number of modes, thereby to produce a more uniform field distribution by the superposition of the various modes. However, the number of modes which can be properly excited are limited by the oven dimensions and the coupling means, and therefore this solution by itself will not always achieve optimum results.

Another proposed solution to the problem of uneven heating of the foodstuffs is to periodically change the field distribution or mode pattern inside the oven. In this method, the position of the nodes and antinodes are continuously changed so that the energy distribution is varied in time to produce a more uniform heating pattern throughout the oven. Heretofore, the field distribution or mode pattern Within the heating cavity has been changed by means of mechanical rotating devices located within the cavity which function to change the boundary conditions of the oven, thereby changing the field distribution of the microwave energy in the cavity. The mechanical stirrer or mode shifter may also be thought of as an element for reradiating or reflecting the microwave energy to provide a more uniform distribution of the microwave energy within the oven. The field stirrer is usually a fan-like device having metal blades which are rotated at slow speeds by a small motor. The stirrer is usually located in the cavity near the port through which the microwave energy enters, and as it rotates the passing blades influence the microwave generator frequency. The mode pattern is thus varied by variations in the microwave generator frequency as Well as by varying the boundary conditions in the oven. Mechanical stirrers have several disadvantages, one of which is the variations in the standing wave ratio produced by the rotating blades. Mechanical stirrers also present the problem of preventing energy leakage through the hole in which the driving shaft projects.

3,366,769 Patented Jan. 30, 1968 It is therefore an object of the present invention to provide an improved microwave oven which eliminates the disadvantages and undesirable features of the known devices.

Another object of this invention is to provide apparatus for improving the efficiency and uniformity of the distribution of microwave energy in a microwave oven without the use of mechanical rotating devices.

A further object is to provide a microwave oven in which dielectric materials whose linear dimensions are large compared to the wavelength of the microwave energy used may be heated in a substantially uniform manner.

A still further object is to provide a microwave energy coupling structure for a microwave oven of simple construction and design and of low cost.

Another object of the invention is to provide a microwave energy coupling structure for a microwave oven which is adjustable to provide a substantially uniform heating pattern within the oven.

An additional object of the invention is to provide a simpleand effective energy coupling system which can be used in conjunction with a variety of coupling antennas to improve the radio frequency energy distribution throughout a microwave oven thereby to provide more uniform heating of a dielectric material.

The present invention achieves the foregoing objects by providing an improved wave energy coupling system for a microwave oven which, in a preferred embodiment of the invention, comprises a hollow cylinder of conductive material which is located in the vicinity of the radiating antenna and serves to provide a more uniform coupling of the microwave energy to the dielectric load than would be possible in the absence of said cylinder.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will become apparent from the following description considered in connection with the accompanying drawings wherein:

FIG. 1 is a vertical sectional view through a microwave oven embodying the invention;

FIG. 2 is a front elevation view of a portion of the oven of FIG. 1 looking at the rear wall thereof along the line 22 of FIG. 1;

FIG. 3 shows an enlarged vertical sectional view of a portion of a second embodiment of the novel oven arrangement;

FIG. 4 shows an enlarged vertical sectional view of a portion of a third embodiment of the novel oven arrangement;

FIG. 5 shows an enlarged vertical sectional view of a portion of a fourth embodiment of the novel oven arrangement; and,

1 FIG. 6 shows an enlarged vertical sectional view of a portion of a fifth embodiment of the novel oven arrangement.

Referring now to FIGS. 1 and 2 of the drawing, there is shown an oven 1 constructed of a suitable electrically conductive material having relatively thin walls and formed in the shape of a hollow enclosure or cavity. A hinged door 2 of electrically conductive material provides access to the cavity. When closed, the door completely seals the cavity 1 so as to prevent the escape of microwave energy to the surrounding environment.

Positioned within cavity 1 there is shown a container 3 of food or other dielectric material to be heated. The container is placed on a shelf 4 which is preferably composed of a low loss dielectric material, for example, polypropylene. The height of the shelf 4 within the oven may be varied by providing suitable support means (not shown) on the side walls of the cavity. Alternatively, the dish 3 may be placed on the bottom of the cavity 1 or upon a block of suitable low loss dielectric material which rests on the bottom of the oven.

A source of microwave energy comprising an electron discharge device 5, which is illustrated .as a magnetron, but may alternatively be of any other type capable of generating and transmitting radio frequency energy to the oven, is mounted externally on the rear wall of the oven. Magnetron 5 has an evacuated envelope 6 of highly con ductive material as, for example, copper, and a plurality of inwardly directed radially disposed anode vanes 7. The arrangement is such that each pair of adjacent anode vanes forms, together with that portion of the envelope lying therebetween, a cavity resonator whose natural resonant frequency is, as is well known to those skilled in the art, a function of the geometry of the physical elements defining it. For the purposes of the present invention, it is desirable that the dimensions of the magnetron be such that the wavelength of the electrical oscillations generated therein is in the centimeter range. Wavelengths of this order are in the microwave region and have been found to be particularly suitable for the dielectric heating of foodstuffs. However, this invention is also useful with ra dio ffrequency energy of longer or shorter wavelengths.

Magnetron 5 is energized from any suitable voltage source (not shown) and, when so energized, delivers high frequency electromagnetic energy to the oven 1 by means of a coaxial transmission line comprising an inner conductor 8 which extends into the magnetron cavity, and an outer conductor 9 which is connected to envelope 6. The inner conductor 8 terminates in a cylindrical antenna member 10 which extends into the interior of oven 1 through a suitable opening in the rear wall thereof. The outer tubular conductor 9 is fastened to the rear wall of the oven by means of a washer 11. A conically shaped insulator 12 separates the inner and outer conductors and provides a vacuum seal. A more detailed description of a magnetron generator suitable for use with the present invention is disclosed in a copending US application, Ser. No. 276,838, filed Apr. 30, 1963 and assigned to the assignee of the present invention.

A particular form of output coupling antenna which has been found to be particularly effective in producing the desired energy distribution pattern within the oven comprises a coupling loop having one end connected to the rear wall 13 of the oven. The coupling loop comprises a thin elongated solid brass member 14 which is rotatably mounted on antenna member 10 at right angles thereto. For the sake of clarity, the means for rotatably mounting member 14 on member 10 are not illustrated, any one of a. variety of known mounting techniques being suitable. A solid brass cylindrically shaped member 15 is mounted on the free end of member 14 at right angles thereto so as to complete the loop. The free end of mem' ber 15 has a lip portion 16 projecting therefrom and a set screw 17 mounted therein for fastening the coupling loop in position against the rear wall of the oven.

When the magnetron 5 is energized, the apparatus thus far described radiates electromagnetic energy into the oven 1. If the oven dimensions are suitably chosen with respect to the frequency of the microwave energy source, a particular complex mode pattern of radio frequency energy is established in the cavity. Although a plurality of modes are excited within the oven, it has been found that a fairly large standing wave ratio nevertheless exists which causes the production of positions of maxima and minima of energy distribution in the oven. As a result, uniform heating of the dielectric material in container 3 is difficult to achieve.

In order to produce a more nearly uniform energy distribution within the oven, a hollow cylinder 18 composed of brass or other suitable metallic material is adjustably mounted on member 14, for example, by means of a screw 19. Cylinder 18 has been found to modify the energy distribution pattern in the microwave oven so as to produce uniform heating of the contents of container 3. Cylinder 18 can be rotated through an angle of 360 degrees.

In operation of the oven, a given sample to be heated is placed in the oven. The loop coupling structure comprising elements 14 and 15 can be rotated about antenna member 10 through an angle of 360 degrees. Adjustment of coupling loop 1415 and cylinder 18 will produce an optimum field configuration within the oven whereby uniform heating is obtained. Once adjusted to this optimum position, set screw 17 and screw 19 are tightened to hold the antenna structure in position. A very uniform. heating pattern is thus produced without the use of complex mode stirrer structures.

If it is desired to heat a substance having sharply different dimensions and dielectric properties, it is only necessary to readjust the relative positions of loop coupling members 14 and 15 and cylinder 18 to produce an optimum field configuration to produce uniform heating within the oven. A very simple yet effective oven for cooking purposes is thus produced.

It is not known exactly how the cylinder 18 coacts with the antenna coupling system to produce the beneficial results described, but several possible theories are suggested. One suggested theory is that the microwave energy established in the cavity is inductively coupled intO cylinder 18 causing current to fiow therein. Cylinder 18 in turn reradiates energy in varying proportions into the various modes existing in the cavity thereby to modify the energy distribution pattern in the oven. In other words, cylinder 18 provides inductive coupling of energy between the various modes established in the cavity to produce a more uniform heating pattern. A second suggested theory is that the cylinder may actually affect the number of modes excited in the cavity in such a manner as to produce a more uniform overall field distribution pattern.

A particular embodiment of this invention which was found to produce uniform heating and an acceptable standing wave ratio consisted of a cube shaped cavity having equal length, width and height dimensions of 10 inches. The microwave frequency source was a magnetron which generated radio frequency energy of 2,450 megacycles/second. Antenna element 10 was mounted on the rear wall of the oven at a point equally spaced from the side walls and consisted of a solid brass rod of /8 inch diameter by 1 /8 inches long. Element 14 was 1 inches long, /8 inch wide and inch thick. Element 15 was a solid brass rod 2 /8 inches long and /s inch in diameter. Cylinder 18 was /8 of an inch long, with a /8 of an inch outside diameter and a wall thickness of of an inch. Element 14 was adjusted and fixed in position at an angle of 45 degrees to the vertical axis of the oven. Cylinder 18 was adjusted and fixed in position so that its longitudinal axis was parallel to the floor of the oven. With the above dimensions and adjustments, a uniform heat pattern was established at various points throughout the oven and an acceptable VSWR established. Therefore, positioning of the dielectric substance to be heated within the oven was not critical, which is a further advantage of the invention.

FIG. 3 is a fragmentary view of another embodiment of the invention. In this figure the oven configuration is similar to that shown in FIG. 1, except that the antenna coupling structure 14 is in contact with the top surface 20 of the oven rather than the rear wall. Cylinder 18 is rotatably mounted on element 14 by means of screw 19, in a manner similar to that illustrated in FIG. 1. In this embodiment, however, the cylinder 18 is mounted along the axis of antenna element 10. The dimensions of the various elements of this system are similar to the dimensions of the first embodiment described above and the system of FIG. 3 operates in a manner similar to that described for FIGS. 1 and 2.

FIG. 4 is a fragmentary view of still another embodiment of the invention. Once again, the oven configuration is generally similar to that shown in FIG. 1. However, in this embodiment, the antenna loop coupling structure 14-15 of FIG. 1 has been dispensed with and a simple linear probe element serves as the basic antenna radiating element. A thin strip of insulation 22 is mounted on the end of element 10. The hollow cylinder 18 is rotatably mounted on the end of antenna element 10 by means of a screw 19, thereby providing means for adjusting the mode pattern within the oven. Insulation strip 22 separates antenna element 10 from cylinder 18.

In FIG. 5 there is disclosed a fragmentary view of another embodiment of the invention. In this figure the oven configuration and microwave energy feed system are similar to that shown in FIG. 1. The antenna structure in this system effectively comprises two loop coupling structures. The first loop consists of elements 14a and 15a connected to the rear wall of the oven. The second loop consists of elements 14b and 15b, also connected to the rear oven wall. Hollow cylinder 18 is rotatably mounted on the lower end portion of element 14b. Cylinder 18 could also be mounted at other points along the element 14, for example, at the upper end of element 14a, directly over antenna element 10, or at other intermediate points along element 14. It is also within the scope of this invention to mount two or more cylinders at various points along element 14. Element 14 may also be rotatably mounted on element 10 to provide further possibility of adjustment of the mode pattern in the oven. In this embodiment of the invention, the coupling cylinder 18 functions in substantially the same manner and provides the same advantages as in the embodiment of FIG. 1.

In the various embodiments of the invention described above, the improved coupling system included a hollow conductive cylinder member 18. Other geometrical configurations of the member 18 will also provide improved heating patterns within the oven. For example, hollow members having a square, rectangular, elliptical or other cross section can be made to work. While hollow members have been found to provide the best results, it is also possible to use a solid member. In particular, a solid cylinder has also been found to provide a certain improvement in the heating pattern in the oven when adjusted to an optimum position. Another particularly advantageous form of the invention is shown in FIG. 6. In this figure, the coupling element 21 comprises a hollow rectangular shaped member composed of a conductive material which is adjustably mounted on the element 14 in a manner similar to that shown in FIG. 1 for the hollow cylinder 18 The hollow rectangular element 21 functions substantially in the same manner and provides the same advantages as the hollow cylinder embodiment of FIG. 1.

In the various embodiments of the invention disclosed in FIGS. 1-6, the adjustable wave energy coupling member has been mounted directly on the antenna structure. It is by no means necessary that this method of mounting be employed. However, since an antenna structure is usually necessary to supply microwave energy to the oven, it was found to be convenient to use this structure for thedual purpose of supporting the coupling loop. It is also possible to mount the coupling cylinder at the end of a ceramic member extending from the top wall of the oven down into the cavity. Furthermore, as suggested in FIG. 4, it is not necessary that the metal cylinder 18 be in direct metal to metal contact with the metal antenna element 14. Equally good results can be obtained by separating the hollow cylinder from the antenna structure, for example, by means of a thin Teflon washer. In this case a nonconductive screw may be used as the fastening element.

Although particular embodiments of the invention have been illustrated and described herein, it is to be understood that various alterations and modifications may be made in the structure shown without departing from the spirit and scope of the invention. For example, the shape and dimensions of the oven may be varied, the type of antenna coupling structure may be varied to solve particular problems, and the geometrical configuration and dimensions of the coupling element may also be varied. Accordingly, the invention is not to be limited to the particular details of the embodiments disclosed herein, except as defined by the appended claims.

What I claim is:

1. Heating apparatus employing high frequency electromagnetic energy of a given wavelength comprising a metallic enclosure defining a cavity having internal dimensions which are larger than said given wavelength whereby said cavity can be excited in a plurality of oscillation mode patterns, said cavity being adapted to contain a body to be heated, means for supplying said high frequency energy to the interior of said enclosure comprising a wave energy radiating element extending into said cavity, a hollow tubular wave energy cou ling member composed of a conductive material and positioned entirely within said cavity at a point at which it can couple a portion of the energy contained in one of said modes to a different one of said modes existing in said cavity thereby to alter the energy distribution pattern within said enclosure, and means for rotatably mounting said tubular member for rotation about a given axis.

2. Apparatus as described in claim 1 wherein said enclosure comprises a substantially rectangular shaped box having a vertical axis and wherein said hollow tubular member is rotatablymounted so that its longitudinal axis can be rotated in a plane parallel to the vertical axis of said enclosure.

3. Heating apparatus employing high frequency electromagnetic energy of a given wavelength, comprising a metallic enclosure defining a cavity having internal dimensions which are larger than said given wavelength, means for supplying said high frequency energy to the interior of said enclosure comprising a wave energy radiating structure extending into said cavity, said radiating structure comprising a first linear element extending at right angles to a wall of said cavity and a second L- shaped element rotatably mounted on said first element and having one leg of said L in contact with said wall, said cavity being adapted to hold a body to be heated by said high frequency wave energy in spaced relationship with said wave energy radiating structure, and means for altering the energy distribution pattern in said enclosure comprising a hollow metallic tube mounted on said L- shaped element.

4. Heating apparatus employing high frequency electromagnetic energy comprising a rectangular shaped enclosure having metal walls defining a cavity, means for supplying said high frequency energy to the interior of said enclosure comprising a wave energy radiating structure extending into said cavity, said radiating structure comprising a first linear element extending at right angles to a wall of said cavity and a second L-shaped element comprising first and second legs, means for rotatably mounting said first leg on said linear element at right angles thereto so that said second leg contacts said cavity wall, said cavity being adapted to hold a body to be heated by said high frequency wave energy at a position in said cavity spaced from said wave energy radiating structure, means for altering the energy distribution pattern within said enclosure comprising a hollow metallic wave energy coupling member having a longitudinal axis, and means for rotatably mounting said hollow coupling member on said first leg so that its longitudinal axis can be rotated in a plane parallel to said cavity wall.

5. Heating apparatus comprising a source of high frequency electromagnetic energy of a given wavelength, an electrically conductive enclosure defining a cavity having internal dimensions which are larger than said given wavelength whereby said cavity can be excited in a plurality of oscillation mode patterns, means coupled to said energy source for supplying said high frequency energy to the interior of said enclosure, said latter means comprising a metallic wave energy radiating structure extending into said cavity thereby to excite said cavity with said plurality of oscillation modes, said cavity being adapted to hold a dielectric body to be heated by said high frequency wave energy at a position in said cavity spaced from said wave energy radiating structure, and means for modifying the mode pattern Within said cavity comprising a metallic wave energy coupling structure mounted on said radiating structure and dimensioned so as to be non-resonant at said given wavelength.

6. Apparatus as described in claim 5 wherein said oven comprises a substantially rectangular cavity having mutually perpendicular top and side walls, said wave energy radiating structure comprising a linear probe mounted parallel to said top wall, and said coupling structure comprises a cylindrical member mounted on said linear probe at right angles thereto.

7. Apparatus as described in claim 6 further comprising an insulation body mounted between and separating said linear probe and said cylindrical member.

8. Heating apparatus comprising a source of high frequency electromagnetic energy of a given wavelength, an electrically conductive enclosure defining a cavity having at least one side wall and a top wall and internal dimensions which are larger than said given wavelength whereby said cavity can be excited in a plurality of oscillation mode patterns, a metallic wave energy radiating structure coupled to said energy source and extending into said cavity for supplying said high frequency energy to the interior of said enclosure thereby to excite said cavity with a plurality of oscillation modes, said cavity being adapted to hold a dielectric body to be heated by said high frequency wave energy at a position in said cavity spaced from said wave energy radiating structure, said radiating structure comprising a first linear member mounted perpendicular to said side wall and a second linear member mounted on said first linear member at right angles thereto in contact with said top wall, and means for modifying the mode pattern within said cavity comprising a metallic coupling structure that comprises a cylindrical member mounted adjacent to said radiatin g structure.

9. Apparatus as described in claim 8 wherein said cylindrical member is hollow and has a longitudinal axis and further comprising means for rotatably mounting said cylinder on said radiating structure so that the longitudinal axis of said cylinder is perpendicular to said first linear member for all rotatable positions thereof.

10. Heating apparatus employing high frequency electromagnetic energy of a given wavelength comprising a metallic enclosure defining a cavity having internal dimensions which are larger than said given wavelength, means for supplying said high frequency energy to the interior of said enclosure comprising a wave energy radiating structure extending into said cavity, said radiating structure comprising a first linear element extending at right angles to a wall of said cavity and a second U- shaped member having a pair of legs and a base portion, means for mounting said base portion on said linear element so that said pair of legs extend toward said wall, said cavity being adapted to hold a body to be heated by said high frequency wave energy at a position in said cavity spaced from said wave ene gy structure, and means for modifying the mode pattern within said cavity to produce a substantially uniform distribution of energy in the region to be occupied by said body and comprising a hollow metallic member mounted adjacent to said base portion of said U-shaped member.

11. Apparatus as described in claim 10 wherein the legs of said U-shaped member contact said cavity wall and said hollow member comprises a cylindrical member rotatably mounted on said base portion.

12. Heating apparatus employing high frequency electromagnetic energy of a given wavelength comprising a metallic enclosure defining a cavity having internal dimensions which are larger than said given wavelength, means for supplying said high frequency energy to the interior of said enclosure comprising a wave energy radiating structure extending into said cavity, said radiating structure comprising a first linear element extending at right angles to a wall of said cavity and a second L-shaped element rotatably mounted on said first element and having one leg of said L in contact with said wall, said cavity being adapted to hold a body to be heated by said high frequency wave energy at a position in said cavity spaced from said wave energy radiating structure, and means for modifying the mode pattern within said cavity comprising a hollow metallic tube having a rectangular crosssection mounted on said L-shaped element.

13. High frequency heating apparatus comprising a source of high frequency electromagnetic energy of a given wavelength, a metallic enclosure defining a cavity having internal dimensions which are larger than the wavelength of said high frequency energy source, a wave energy radiating structure extending into said cavity, means for coupling said high frequency source to said radiating structure thereby to excite said cavity in a plurality of oscillation mode patterns, said cavity being adapted to hold a body to be heated by said high frequency energy, and means for modifying the mode pattern within said cavity comprising a non-resonant metallic wave energy coupling member mounted on said radiating structure within said cavity at a point at which it can effectively couple said energy between different ones of said oscillation modes existing in said cavity.

14. Apparatus as described in claim 13 further comprising means for electrically connecting said coupling member to a wall of said enclosure. I

15. Apparatus as described in claim 13 wherein said radiating structure comprises a first linear member composed of a conductive material, said coupling member comprising a second linear member mounted perpendicular to said first member and electrically connected thereto, the length dimension of said second member being independent of the wavelength of said high frequency energy source.

16. Apparatus as described in claim 15 wherein said second linear member is adjustable to a plurality of positions all of which lie in a plane perpendicular to said first linear member.

17. Apparatus as described in claim 13 wherein said radiating structure comprises a first linear metallic member and a second linear metallic member mounted on said first member at right angles thereto, and said coupling member comprises a third linear member mounted on said second member with its longitudinal axis perpendicular to said first member.

18. Apparatus as described in claim 17 wherein said third member is adjustable, said third member being rotatably mounted about an axis transverse to its longitudinal axis.

19. Apparatus as described in claim 18 wherein at least one of said first and second members is adjustable to a plurality of fixed positions within said enclosure.

20. Apparatus as described in claim 17 wherein said third linear member comprises a hollow metallic tube.

21. A microwave heating oven comprising a source of microwave energy adapted to operate at a given wavelength, a heating chamber having a conductive wall structure dimensioned to support a plurality of resonant modes at said given wavelength, a primary wave energy radiating structure positioned within said chamber, said radiating structure being substantially the sole source of primary microwave energy radiation within the chamber, means for coupling said microwave source to said ber in said plurality of resonant modes, and a non-resonant secondary conductive structure comprising an elongated metallic member having a continuous outer wall completely radially surrounding an axially extending aperture and positioned entirely within said chamber and in the vicinity of said primary radiating structure thereby to modify the energy pattern produced by said primary radiating structure so as to provide a substantially uniform References Cited UNITED STATES PATENTS KrayeWski 21910.55 De Bell 21910.55 Long 21910.55 Johnson 219-10.55 Staats 219-10155 Crapuchettes 21910.55

energy distribution pattern over a large area of said 19 RICHARD M-WOODP"imary Examine"- L. H. BENDER, Assistant Examiner.

chamber. 

1. HEATING APPARATUS EMPLOYING HIGH FREQUENCY ELECTROMAGNETIC ENERGY OF A GIVEN WAVELENGTH COMPRISING A METALLIC ENCLOSURE DEFINING A CAVITY HAVING INTERNAL DIMENSIONS WHICH ARE LARGER THAN SAID GIVEN WAVELENGTH WHEREBY SAID CAVITY CAN BE EXCITED IN PLURALITY OF OSCILLATION MODE PATTERNS, SAID CAVITY BEING ADAPTED TO CONTAIN A BODY TO BE HEATED, MEANS FOR SUPPLYING SAID HIGH FREQUENCY ENERGY TO THE INTERIOR OF SAID ENCLOSURE COMPRISING A WAVE ENERGY RADIATING ELEMENT EXTENDING INTO SAID CAVITY, A HOLLOW TUBULAR WAVE ENERGY COUPLING MEMBER COMPOSED OF A CONDUCTIVE MATERIAL AND POSITIONED ENTIRELY WITHIN SAID CAVITY AT A POINT AT WHICH IT CAN COUPLE A PORTION OF THE ENERGY CONTAINED IN ONE OF SAID MODES TO A DIFFERENT ONE OF SAID MODES EXISTING IN SAID CAVITY THEREBY TO ALTER THE ENERGY DISTRIBUTION PATTERN WITHIN SAID ENCLOSURE AND MEANS FOR ROTATABLY MOUNTING SAID TUBULAR MEMBER FOR ROTATION ABOUT A GIVEN AXIS. 